cold head running · helium returns as liquid
Liquid helium, 4.2 K: level is liveSuperconducting coil, in the heliumChilled water: compressor and gradient amplifierHelium gas lines, cold head to compressor5 gauss line (schematic)Cut at the mid-plane, from outside in: GRP cover, steel vacuum vessel, copper thermal shield, helium vessel with the superconducting coil in liquid helium, then the gradient coil and the RF body coil in the bore. The field never switches off. The cold head on top runs on helium gas from the compressor and turns the boiled-off helium back into liquid, so a healthy magnet loses almost none. Stop the compressor and the level starts to fall: the helium is not used up, it stops coming back. Time on this figure runs at one hour per second; the boil-off rate (about 10 % a day once the cold head is warm) is illustrative, of the order of a reported case where a magnet lost about 1,200 litres in a week with a failed cold head. The chiller outdoors removes the heat of the compressor and the gradient amplifier: when it stops, the compressor stops too.